1. Field of the Invention
The present invention relates to a detector for detecting a cardiac event in an IEGM using a bank of digital filters.
2. Description of the Prior Art
Reliable cardiac event detection is of fundamental importance for the control of implantable heart stimulators, such as pacemakers and defibrillators. An increasing problem in today's society in this connection is the amount of noise sources which to various degree interfere with implantable devices, see e.g. W. IRNICH, “Interference in Pacemakers”, PACE, vol. 7, pp. 1021–1048, November–December 1984. During the last decades an increasing number of different types of electrical equipment has been introduced to the general public, which may be potential threats to the correct function of implantable devices. Moreover, internal muscle noise interferes with pacemakers, see W. Irnich “Muscle Noise and Inference Behaviour in Pacemakers: A Comparative Study”, PACE, vol. 10, pp. 125–132, January–February 1987.
During the last twenty years a large number of surface electrocardiogram ECG based QRS detectors have been presented for processing surface ECG:s. Many of these QRS detectors exhibit the same basic structure, viz. linear filtering and a non-linear transformation of the signal for the subsequent decision rule, see e.g. O. Pahlm and L. Sörnmo, “Software QRS detection in ambulatory monitoring—a review”, Med. Biol. Eng. Comput., vol 22, pp. 289–297, 1984. QRS detectors implemented in implantable medical devices exhibit a similar structure but typically involve only a bandpass filter and an amplitude threshold. However, cardiac event detection using intracardiac electrograms IEGM introduces new demands disqualifying most ECG based QRS detectors. In an implantable heart stimulator the detection decision must be made in real time with a delay of no more than approximately 40 ms. Furthermore, implementation for implantable medical devices introduces power constraints, which in general make use of ECG based QRS detectors impossible. The evolution of digital integrated circuits has allowed for more complex digital detectors and more advanced implantable medical devices. The use of digital detectors also has the advantage of simplifying the implantation of other desirable features such as morphology classification and data compression for post analysis. PCT Application WO 0072916 describes an implantable stimulator that subjects cardiac signals to different filtering. The signals from the different filters are then combined and compared with predetermined criteria to determine if the sought cardiac signal is present or not.
Heretofore, however, digital application specific integrated circuits have been a non-realistic alternative for cardiac event detection in implantable medical devices because of the power demands. There is thus a need for better possibilities to detect cardiac events in implantable medical devices.